Comtech EF Data SMS-458B - Manual (Page 66)

SMS-458B Modem Protection Switch Revision 2
Theory of Operation MN/SMS458B.IOM
4–6
4.1.4.1 Scope
This section defines the protocol and the word and command structure of the relay-
remote interface.
4.1.4.2 General
The relay-remote input (J6) allows the user to execute a limited number of mode and
online commands (refer to Section 2.2.5 for connector location and pinout).
These commands are a series of 4-bit words. The switch has a 4-line parallel interface
with pull-up resistors (10 K
Ω
to +5V). Thus, open inputs are logic high, and connecting
to pin 25 of J6 can generate logic low inputs. The inputs can also be driven directly from
TTL or CMOS family devices.
The M:N switch recognizes a word 100 msec after the change of state is detected on the
parallel interface. The words must be held for 200 msec, and should not be sent faster
than 500 msec or slower than 10-second intervals. The word can be repeated any number
of times, but the switch only detects it the first time it is sent. The following section
describes the commands that can be executed.
4.1.4.3 Command Structure
A command to the switch consists of three or four words sent on the four parallel lines in
one of the following formats:
• Non-return-to-zero
(NRZ)
• Return-to-zero
(RZ)
The command sequence begins with the START word. The switch recognizes each word
by changes of state at the relay-remote inputs. After debounce, the switch identifies the
word. If all words are valid and the EXECUTE word is received, the switch performs the
command.
The command words START and EXECUTE are defined in Table 4-1.